This invention relates to an apparatus for forming a thin film on a substrate, and more particularly, it relates to an apparatus which can form multi-layer films employing reactive sputtering.
A conventional film-forming apparatus of the type to which the present invention relates has a single reaction chamber which houses an anode and a cathode between which a direct current voltage is applied. The anode serves as a support for a substrate on which a thin film is to be formed. After evacuating the reaction chamber, a rare gas (typically argon) is introduced into the reaction chamber, and a gaseous discharge is produced by applying a high voltage between the electrodes. The rare gas ions which are formed by the gaseous discharge strike the cathode, and particles of the material which constitutes the cathode are sputtered into the reaction chamber and accumulate as a thin film on the substrate.
Reactive sputtering is a form of sputtering used to form thin films of chemical compounds such as oxides or nitrides on a substrate. Both a rare gas such as argon and a reactive gas such as oxygen or nitrogen are introduced into an evacuated reaction chamber. As in the above-described method, a high voltage is applied between a cathode and an anode, which supports a substrate, and rare gas ions which strike the cathode cause particles of the material constituting the cathode to be sputtered from therefrom. The sputtered particles combine with the reactive gas to form a compound, which then accumulates on the substrate as a thin film.
It is often desired to form a multi-layer thin film on a substrate. For example, in one type of multi-layer film, a first layer of a chemical compound (which will be referred to as a chemical compound thin film) is formed on a substrate by reactive sputtering, and then a second layer of a material in elemental form (which will be referred to as an elemental thin film) is formed atop the first layer using some other film-forming method. When using a conventional film-forming apparatus, after the chemical compound thin film is formed on a substrate by reactive sputtering in a reaction chamber, the gas in the reaction chamber must be completely evacuated prior to forming the elemental thin film, since if any of the reactive gas remains in the reaction chamber while the elemental thin film is being formed, the reactive gas will end up in the elemental thin film and will adversely affect its properties. However, evacuating the reaction chamber between the steps of forming the two layers is troublesome, time-consuming, and decreases productivity.
Furthermore, because of the need to evacuate the reaction chamber before forming the elemental thin film, the chemical compound thin film is left exposed in a vacuum for a considerable length of time. The exposure decreases the stability of the interface between the chemical compound thin film and the elemental thin film, often resulting in the undesirable peeling or swelling of the elemental thin film.